An air-fuel ratio sensor may typically add a relatively small additional delay/lag to a feedback signal due to the sensor's protective covering and the time required for electro-chemical processing. A degraded sensor, possibly one where its protective covering is contaminated, may add more delay/lag. For example, the degraded sensor signal may be either delayed (but otherwise the same as the actual signal) or filtered (spread out in time with a reduced amplitude of the actual signal). In such cases, a feedback controller may not operate as desired due to higher than expected delay/lag.
In one example, to compensate for such delay/lag, the air-fuel controller may include a predictive delay compensation control structure, such as a Smith Predictor. The Smith Predictor may allow the controller to regulate the continuous dynamics of the system through a feed forward mechanism that compensates for delay/lag when the measured signal differs from the Smith Predictor's estimate.
However, the inventors have recognized several potential issues with such an approach. For example, the accuracy of the predictive delay compensation control structure may be affected by non-linear air-fuel ratio sensor degradation. For example, the predictive delay compensation control structure creates a bias for asymmetric faults in which a delay or filter lag is imposed on one direction of air-fuel ratio transition (e.g., lean to rich or rich to lean) but not the other direction. In particular, the bias leads to corrective overshoot and other feedback control errors, even if offsets are provided when the asymmetric air-fuel ratio sensor faults are identified. Such feedback control errors result in an increase of emissions of regulated gases NOx, CO, and NMHC.
The inventors herein have identified an approach for mitigating the bias in order to increase feedback control accuracy when an asymmetric fault of an air-fuel ratio sensor is identified. In one embodiment, a method includes adjusting a structure of the air-fuel controller to mitigate the delays caused by an asymmetric fault, rather than adjust an offset or gain parameters.
In one example, a method includes adjusting fuel injection to an engine responsive to air-fuel ratio sensor feedback with a first control structure. The method further includes in response to air-fuel ratio sensor asymmetry degradation, adjusting fuel injection to the engine responsive to air-fuel ratio sensor feedback with a second, different, control structure. In particular, the first control structure includes a Smith Predictor delay compensator that is dependent on linear dynamic operation of the air-fuel ratio sensor for suitable control accuracy. Further, the second control structure includes an internal model of behavior of the air-fuel ratio sensor degradation. The internal model may include a model of the actual asymmetric behavior of the degraded air-fuel ratio sensor. Accordingly, the controller provides accurate delay compensation via the Smith Predictor during dynamic linear operation and maintains control accuracy in response to identifying non-linear asymmetric operation by switching to an internal model that compensates for the asymmetric behavior. In this way, both the bias and the overshoot that would be caused by the Smith Predictor due to the asymmetric fault may be eliminated.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.